In general, a holding valve that is mounted on a main control valve (MCV) of a construction machine is used to prevent a self-weight descent of a working device such as a boom, due to leakage of hydraulic fluid through a gap between a spool and a valve block in a neutral position of the working device, such as a boom, and to control the hydraulic fluid that is supplied to a hydraulic actuator when the working device is operated.
As illustrated in FIGS. 1 to 3, a pressure control valve for a construction machine in the related art includes a holding valve block 4 having a pilot signal pressure port 1 and a drain flow path 2 formed thereon and installed between a hydraulic actuator (boom cylinder or the like, not illustrated) and a control valve 3, a holding poppet 6 having a back pressure chamber 5 formed therein to intercept leakage of hydraulic fluid from the hydraulic actuator to the control valve 3 in a neutral state, a first elastic member 7 pressing the holding poppet 6 and elastically biasing an intercepted path between the hydraulic actuator and the control valve 3 in an initial state, a piston 8 sliding in the holding valve block 4 when pilot signal pressure is input through the pilot signal pressure port 1, a pilot poppet 11 shifted along an inner surface of a sleeve 9 of the holding valve block 4 to drain the hydraulic fluid in the back pressure chamber 5 toward a drain flow path 10 of the control valve 3 in response to movement of the piston 8, and a second elastic member 12 pressing the pilot poppet 11 and elastically biasing an intercepted path between the back pressure chamber 5 of the holding poppet 6 and the drain flow path 10 of the control valve 3 in an initial state.
According to the pressure control valve in the related art as configured above, if a spool 3a of the control valve 3 is shifted in a rightward direction as shown in FIG. 2 by applying a control signal through an operation of an operation lever (RCV lever) to drive boom-up or arm-out, hydraulic fluid that is discharged from the hydraulic pump 13 moves through a parallel path 14 and a flow path 15 to lift the holding poppet 6 in an upward direction in the drawing. In this case, hydraulic fluid in the back pressure chamber 5 flows toward a cylinder port 16 through an orifice 19.
Through the movement of the holding poppet 6, the flow path 15 and the cylinder port 16 on the side of the hydraulic actuator communicate with each other, and thus the hydraulic fluid that is discharged from a hydraulic pump 13 is supplied to the hydraulic actuator to drive the hydraulic actuator to perform boom-up operation. In this case, the drain flow path 10 of the control valve 3 that communicates with the flow path 15 is closed by a check valve 17 of the holding valve block 4.
Since an excavator or the like mainly operates in a work environment having poor work conditions due to the equipment characteristics, fine foreign substances, such as dust, exert a bad influence on valve seat surfaces of various kinds of control valves. As an example, if foreign substances stick to a valve seat surface 17a of the check valve 17, a sealing function is not normally performed between the check valve 17 and the valve seat surface 17a to cause leakage of the hydraulic fluid.
If a check function of the check valve 17 is not performed, the hydraulic fluid, which is supplied from the hydraulic pump 13 to the flow path 15, moves to a port 18 on the side of the pilot poppet 11 via the drain flow path 10 and the check valve 17.
Since the pilot poppet 11 moves along the sleeve 9 in a downward direction in the drawing by the hydraulic fluid that is supplied to the port 18 (in the case where the hydraulic fluid pressure that is applied to the pilot poppet 11 exceeds an elastic force of the second elastic member 12), the port 18 and the back pressure chamber 5 in the holding poppet 6 communicate with each other. That is, under normal conditions, the pilot poppet 11 moves in a downward direction only by the movement of the piston 8, but it moves abnormally in the downward direction due to an inferior sealing function of the check valve 17.
On the other hand, a pressure receiving area of the holding poppet 6, to which pressure is applied in the back pressure chamber 5, becomes relatively larger than the pressure receiving area of the holding poppet 6 in a region in which pressure is gone through the orifice 19. Due to this, the pressure in the back pressure chamber 5 is gradually increased to move the holding poppet 6 in the downward direction in the drawing with the lapse of time, and thus the path that makes the flow path 15 and the cylinder port 16 communicate with each other is intercepted.
Accordingly, in the case of performing a boom-up operation of a boom or an arm-out operation of an arm, the operating speed of the hydraulic actuator is lowered, and an excessive load is continuously generated in a hydraulic circuit of the equipment. Since this causes a loss of hydraulic energy, work efficiency is deteriorated.